Falling-film evaporation is a quite common operation, exploited for various purposes; one example being the evaporation of essential oils, in order to separate the active components (from certain terpenic impurities) or the concentration of (NH.sub.4)NO.sub.3 solutions; another example being distillation of urea solutions.
A process for the synthesis of urea (I.D.R. process) is described, for instance, in U.S. Pat. No. 4,208,347 and in Italian patent publication No. 22073 A/83. Said patents state clearly that if one wants to reach high conversion yields for the reaction: ##STR1## one shall use, within the reactor, NH.sub.3 :CO.sub.2 molar ratios much higher than 2; therefore the NH.sub.3 excess, which is present within the synthesis solution (together with the non reacted carbamate), has to be displaced, after the reaction, from the urea solution, usually by means of one or more strippers, whereto heat is supplied, optionally together with the injection of a gaseous stripping agent (coadjuvant), like CO.sub.2 or N.sub.2, and in subsequent steps, frequently isobaric with the synthesis reactor.
The resulting stripped vapors are recycled as such, or condensed in the presence of suitable amounts of water, the solution thus obtained being then recycled to the reactor. After a first high-pressure loop (frequently isobaric with the synthesis) there is a second, and final, low-pressure loop, the so-called "finishing", where the working pressures are very low and where the task is to remove from the solutions, in the vapor phase and as completely as possible, the CO.sub.2 and the NH.sub.3 not bound in the form of an urea molecule (CO.sub.2 being present, e.g., as ammonium carbamate or carbonate). Usually said "non-bound" NH.sub.3 is in strong excess with respect to the "non-bound" CO.sub.2 and with respect to the stoichiometry of ammonium carbamate (NH.sub.3 /CO.sub.2 =0.77 b.w. namely 2:1 by moles)), and the recovery of NH.sub.3 (from the vapors released by the solutions) requires a huge amount of water, which water is necessarily recycled to the synthesis reactor, where an increase of the H.sub.2 O/CO.sub.2 ratio heavily impairs the conversion yields.
It is known that said NH.sub.3 excess (with respect to the carbamate stoichiometry) can only be removed with much more difficulty than the NH.sub.3 and the CO.sub.2 chemically bound in the form of carbamate; therefore an NH.sub.3 :CO.sub.2 ratio of 0.77 b.w., namely a 2:1 ratio by moles, would represent an optimum and desirable choice, independent of the amounts of carbamate, the removal of which carbamate being notoriously a problem far easier to solve.
In other words, especially at the highest pressures (frequently coinciding with the synthesis pressure), it is desirable to suppress nearly completely said NH.sub.3 excess and to keep said ratio near to the optimum level (0.77 b.w.), while committing the "finishing" the easy task to remove the residual carbamate, whatever the amount. Such a suppression is particularly difficult when the stripping is performed without the help of gaseous stripping agents from outside; in such a case, in fact said ratio tends to a considerable increase.
An object of the invention is to look for a stripper in which said NH.sub.3 excess be removed as far as possible and in which said NH.sub.3 :CO.sub.2 ratio be kept as near as possible to the optimum level; other objects will be evident from the description hereinafter.